Films find frequent use in plasters and first aid dressings on account of their imperviousness to water and to microbes, their conformability, and their high level of compatibility.
Accordingly, DE 43 14 834 A1 discloses a film-based dressing material covered on one side with a backing material whose size is the same as that of the film and which has at least one grip strip, and on the other side is provided with a self-adhesive layer. Essential to the invention here is that the grip strips are disposed within the peripheral boundary of the backing material. There is preferably only one grip strip on the backing material. A plaster of this kind with a polyurethane film is available commercially under the name xe2x80x9cAqua Protectxe2x80x9d(copyright) from Beiersdorf.
DE 40 26 755 A1 discloses a film-based dressing material covered on one side with a support material whose size is the same as that of the film and which has at least one grip strip, and on the other side is provided with a self-adhesive layer. In contrast to the dressing material of DE 43 14 834 C2 the grip strips for removing the backing material are disposed within the peripheral boundary of the backing material. Here too, there is preferably only one grip strip on the backing material. This plaster with a polyurethane film is available commercially under the name xe2x80x9cCutifilmxe2x80x9d(copyright), again from Beiersdorf.
Medical plasters, wound dressings, dressings, and fixings of all kinds are often subject to a phenomenon that leads to premature, unintended detachment. This phenomenon is turnup, where the product rolls back starting usually from one corner or else one edge of the plaster.
Once the plaster has come away at one point, there follows a chain reaction which leads very rapidly to complete detachment. With particular frequency, this turnup occurs with plasters worn under clothing or inside footwear. The reason is the rubbing (friction) of the clothes or shoe on the surface of the plaster. This frictional force gives rise to a dynamic shear load on the pressure-sensitive adhesive composition, which usually leads very rapidly to breaking of the bond in the edge region. After the adhesive composition has been released at one edge, the textile or leather clings to the projecting composition and, as a result of the tangentially bearing force, causes turnup and further, accelerated detachment of the whole plaster.
One way of preventing premature detachment is to increase the adhesion of the pressure-sensitive adhesive composition to the skin. This tackiness cannot, however, be increased ad infinitum, since otherwise there may be skin irritation, pain, and disturbance of the wound in the course of the intended detachment of the product.
From EP 0 409 587 A1 it is known to what extent the premature detachment of plasters is affected by the contact area A of the backing film, i.e., the area over which two sliding bodies are actually in contact.
It describes the use of thermoplastic films which during or after the extrusion in the melted state are embossed by an embossing roller. Best results are obtained with a structure in which the contact area represents approximately 25% of the total area.
Using this process, different surfaces can be produced only by employing different embossing rollers. This requires the corresponding rollers first to be produced, in complex and costly processes. A further disadvantage of the process is the generation of positive/negative structures on both sides of the backing. This structure imposes exacting demands in the context of the partial coating with adhesive composition and the placing and anchoring of wound contact materials. The production of multilayer films from hard and soft starting materials is not described. The use of multilayer films to form plasters produced by coating polymer dispersions or polymer solutions onto embossed papers or films is not a subject of the disclosure.
EP 0 446 431 outlines how using casting papers comprising polymer solutions it is possible to produce backing materials for medical use. Subject matter of this invention is, however, a laminate comprising polymeric film layers and a macroporous textile material. The casting paper used in accordance with one example is not described in more detail. Nor is there any indication of the improvement of slip properties by optimizing the contact area and the elasticity of the outer layer, achieved by using casting papers having a structured surface and starting materials of different hardness.
U.S. Pat. No. 5,643,187 describes a film plaster whose backing film consists of two layers, the outer layer of the film being relatively hard, thin, and slippery and the inner layer being relatively soft and thick. Through this combination of two different materials, a film backing having improved slip properties and adequate stability is said to be obtained. This document outlines exclusively the use of film backings having smooth, unstructured surfaces. The improvement of slip properties by optimizing the contact area is not a subject of the description.
A disadvantage of this method of improving the slip is an unavoidable reduction in stretchability, elasticity, and hence conformability of the backing film. Considerably greater force must be applied in order to stretch hard films or film layers, so that in a plaster application there may be instances of skin irritation and a delayed healing process owing to mechanical loading of the wound.
From DE 197 06 380 it is known that a film having direction-dependent water vapor permeability may be produced by multilayer construction. Coextrusion of thermoplastic polyurethanes having different water vapor permeabilities gives a multilayer film which in turn exhibits different permeabilities to water vapor, depending on which side of the film is facing the moisture source. The invention likewise describes the use of polyurethane layers which may differ not only in water vapor permeability but also in their hardness; optimizing slip properties by varying the contact area of the films, however, is not described.
WO 98/41590 describes the production of films based on polystyrene block copolymer/polystyrene blends by melt extrusion, said films featuring good severability and being simple to make available from a dispenser pack. Production and properties, especially slip properties and stretch properties, of films comprising two or more layers of different hardness are not a subject of the overall disclosure.
According to the first law of friction, the frictional force Ff is equal to the product of friction coefficient xcexc and normal force Fn. This coefficient is a measure of the force that must be used to move a body on a surface, xcexcs denoting the static and xcexck the kinetic (sliding) friction coefficient.
The development of backings having good slip properties, i.e., low friction coefficients, is therefore a central starting point for preventing the turnup effect outlined above. Although to date, as set out in particular by Ludema (Ludema, K. C., Friction, Wear, Lubrication: a Textbook in Tribology, CRC Press, Boca Raton 1996), neither exact nor approximate methods exist for deriving friction or wear properties from fundamental principles, an inspection of the literature permits conclusions to be drawn about parameters which determine the size of xcexcs and xcexck.
Static friction is governed (Blau, P J.; Friction Science and Technology, Marcel Dekker, New York 1996) by the following expression:
xcexcs=(xcfx84m/P*)A
where xcfx84m is the shear strength,
A is the contact area, and
P* is the combination of normal force and adhesion.
Sliding friction between two bodies is determined by a range of interacting effects (Bhushan, B., Gupta, B. K.; Handbook of Tribology, McGraw-Hill New York 1991). Besides adhesion components, there occur plowing effects, roughness effects, deformation effects, and, particularly in the case of viscoelastic materials, damping effects. The relative contribution of these effects depends on the materials involved, the surface topography, the state of the sliding surfaces, and the ambient conditions.
Investigations by Bartenev (Bartenev, G. M., Lavrentev, V. V.; Friction and Wear of Polymers, Elsevier Amsterdam 1981) and Rabinowicz (Rabinowicz, E.; Friction and Wear of Materials, Wiley-Interscience, New York 1995) reveal the friction coefficient xcexck to be determined not only by contact area but also by parameters such as roughness, hardness, elasticity modulus, and surface energy of the materials.
The hardness of a plastic is generally stated as the Shore hardness. In the testing of elastomers and rubber, in accordance with DIN 53505 1987-06, the Shore hardness corresponds to the resistance to the penetration of a frustum (A or C) or, respectively, of a rounded cone (D) which is measured by the compression of a spring having a defined spring characteristic and is expressed in dimensionless Shore A (C, D) hardness units. For the testing of steel, the Shore rebound hardness is measured in what is known as a scleroscope, which determines the rebound height of a hammer that falls on the test area in a vertical tube. This is more an elasticity test, and for that reason the measurements are of only limited comparability with those from a Brinell or Vickers hardness test (Rxc3x6mpp Lexikon Chemiexe2x80x94Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998). As may be derived from the above equations, there is a direct correlation between the hardness and slip of a material.
For the conformability and wear comfort of a film plaster, in contrast, the softness, stretchability, and elasticity of the backing film are of central importance. If excessive forces are needed to stretch the backing material while being worn on the skin, caused by body movements such as flexing of limbs or tautening of muscles, these high forces lead to a feeling of tightness and of instances of skin irritation. Proven as a suitable parameter for determining the conformability of a film backing during application on the skin is the 100% modulus. This figure indicates the tensile stress needed to stretch the film by 100%, and may be determined in accordance with DIN 53 504.
It is an object of the invention to avoid the disadvantages known from the prior art and to provide a film plaster which is self-adhesive on one side, possesses good elasticity and stretchability, and owing to improved slip properties does not release unintentionally from the skin side to which it has been bonded previously.
This object is achieved by means of a film plaster as specified in the main claim. The subclaims relate to advantageous developments of the film plaster.
Accordingly, the present invention provides a film plaster, especially for covering wounds and preventing or treating blisters, which comprises a two-layer elastic film, the upper layer being harder and thinner than the lower layer, and being structured, and the surface of the lower layer being coated, where appropriate, with a pressure-sensitive adhesive composition.
In a first preferred embodiment, between the upper layer and the lower layer there is at least one further layer which serves, inter alia, to improve the imperviousness to microbes.
In accordance with the invention, the structure of the upper layer is understood to be a raised patterning, so that the layer is not constructed flatly but instead has three-dimensional elevations and depressions. Alternatively, in one preferred embodiment, the upper layer may comprise individual discrete (i.e., separate) segments.
The film plaster therefore has a three-dimensionally configured surface which is formed by the upper layer or outer layer and which in turn is thinner and harder than the lower layer and than other layers which face the adhesive composition. Surprisingly, by optimizing the structure and hardness of the outer layer, films having improved slip and good conformability and elasticity are obtained.
The surface structure of the films may be laid down in particular by the choice of casting substrate. Preferred casting substrates are those having a structure in which individual elevations are completely separated from one another by depressions. The layer construction of the film may be controlled so that only these elevations are composed of hard, slippery material. The polymer concentration in the dispersion or solution and the width of the gap during coating of the casting substrate determine the thickness of the hard outer layer.
When highly dilute solutions or dispersions, using wetting agents were appropriate, are coated using a low gap width, film embodiments are obtained in which even the elevations consist largely of soft polymer.
Further layers of the film backing of the invention are produced by applying soft, elastic, polymer types.